This invention relates to a method for manufacturing a bump on a terminal face of a semiconductor substrate, in which the terminal face is nucleated to generate a basic metallization through electrolytic coating of the terminal face with zincate, in such a way that zinc particles electrolytically deposited on the terminal face serve as nuclei for an ensuing contact metallization autocatalytically deposited on the basic metallization. In addition, the invention relates to the structure of a bump with a basic metallization situated on a terminal face of a semiconductor substrate and a contact metallization situated on the basic metallization.
It is known in the art for preparing semiconductor substrates for a subsequent bonding to provide the usually aluminum terminal faces of the semiconductor substrates with an elevated contact metallization; socalled xe2x80x9cbumpsxe2x80x9d. In particular when applying the contact metallization to the terminal faces through an autocatalytic deposition of metal, it has proven necessary, in order to improve the adhesion of the autocatalytically deposited metal to the terminal face, not to apply the contact metallization directly to the terminal face, but rather to provide a basic metallization between the terminal face and the contact metallization. This holds true in particular when the contact metallization consists of the autocatalytically deposited metals nickel and gold.
To improve adhesion between the autocatalytically deposited metals and the terminal face, it has proven beneficial to electrolytically coat the terminal face with zincate prior to autocatalytic deposition. The zincate or zinc in the zincate here preferably deposits at the grain boundaries of the relatively coarse-grained aluminum of the terminal face, and thereby produces a relatively coarse distribution of nuclei on the terminal face. This nucleation yields a good adhesion of a metal then autocatalytically deposited on the terminal face. However, the provided coarsely distributed nucleation only results in a correspondingly coarse-grained structure in the autocatalytically deposited metal. This relatively coarse structure facilitates the formation of structural defects, which ends up limiting the reliability of contact metallizations formed in this way.
It is known as an alternative to the electrolytic coating of the terminal faces with zincate to subject terminal faces to electrolytic coating with palladium in preparation for a subsequent autocatalytic metal deposition. The nucleation of the terminal faces achieved as a result has a significantly refined structure with a comparatively higher number of nuclei. This also enables a correspondingly fine-grained formation of the subsequently autocatalytically deposited metal structure. The disadvantage to a palladium nucleation, however, is that the pronounced catalytic property of palladium gives it only a limited selectivity, so that intensive nucleation with palladium can result in a deposition of palladium not just on the terminal faces, but also on the entire surface of the semiconductor substrate. In addition, a palladium nucleation only enables a slight adhesion to a subsequently autocatalytically deposited metallization in particular in comparison to zincate nucleation.
German Laid-Open Patent Specification DE 196 31 565 A1 describes a method in which improved contact metallizations are formed on the terminal faces of a chip by initially applying a flat zinc coating on the terminal face, after which the zinc layer applied in this way is covered with a flat palladium layer.
Patent, Abstracts of Japan No. 02002132 A describes a method for forming soldering bumps in which a flat intermediate layer comprised of a palladium/zinc alloy covering the terminal faces of a chip is formed on the terminal faces of the chip prior to the formation of the soldering bumps.
Patent Abstracts of Japan No. 09283557 A describes a method in which flat layers covering the terminal faces are applied prior to the formation of soldering bumps on the contact surfaces of a semiconductor substrate, wherein the layers each consist of three layers of varying materials. The three layers consist of a palladium/metal alloy layer, a nickel layer and another palladium/inetal alloy layer, wherein zinc, lead, arsenic, bismuth, tin, antimony, tellurium, nickel, cobalt, copper, iron, manganese, gold, quicksilver, silver, cadmium and sulfur are proposed as the metal.
EP 0 308 971 A2 describes a method in which an initial flat layer of palladium followed by a flat layer of nickel is applied to the contact surface prior to the formation of a soldering bump on the terminal face of a semiconductor substrate.
U.S. Patent Specification 5,182,006 describes a method for the formation of flat metal surfaces on zinc-coated aluminum substrates.
The object of this invention is to provide a method for manufacturing a bump with a contact metallization on the terminal face of a semiconductor substrate that makes it possible to further improve the quality of the contact metallization.
According to the invention, a method for manufacturing a bump on a terminal face of a semiconductor substrate is provided, in which the terminal face is nucleated to generate a basic metallization through electrolytic coating of the terminal face with zincate, in such a way that zinc particles electrolytically deposited on the terminal face serve as nuclei for an ensuing contact metallization autocatalytically deposited on the basic metallization. In addition to the electrolytic coating with zincate, an electrolytic coating of the terminal face with palladium takes place, in such a way that, in addition to zinc particles, palladium particles deposited on the terminal face serve as nuclei for the contact metallization subsequently autocatalytically deposited on the terminal face. The basic metallization is formed by numerous independent zinc particles and palladium particles. metallization subsequently autocatalytically deposited on the terminal face. The basic metallization is formed by numerous independent zinc particles and palladium particles.
In addition to the electrolytic coating of the terminal face with zincate, the method according to the invention enables an electrolytic coating of the terminal face with palladium, in such a way that palladium particles deposited on the terminal surface along with the zinc particles serve as nuclei for the contact metallization subsequently autocatalytically deposited on the terminal surface. The basic metallization takes the form of numerous independent zinc particles and palladium particles.
The combination of an electrolytic zincate coating and an electrolytic palladium coating according to the invention results in a hybrid nucleation, which yields a contact metallization that adheres well to the terminal face, and has a fine-grained structure along with a correspondingly smooth surface morphology. This produces a contact metallization with an increased reliability.
In a preferred variant of the method, the electrolytic coating of the terminal face with zincate (zincate coating) is performed in one method step, followed by the electrolytic coating of the terminal face with palladium (palladium coating) in a subsequent method step. In this variant, the subsequent palladium coating refines the nucleation structure produced by the zincate coating.
In particular having zincate coating take place at least partially after a palladium coating yields the advantageous effect that the relatively high erosion rate on the terminal face caused by the zincate coating is reduced by the comparatively low erosion rate of the palladium. This effect also has a beneficial effect on the surface morphology of the contact metallization, since an even greater surface smoothness can be achieved for the contact metallization.
One way to achieve this effect involves performing zincate coating not in a single method step, but interrupting zincate coating and interjecting a palladium coating, so that the zinc particles are at least partially removed prior to palladium coating, followed by repeated zincate coating.
Another possibility has to do with electrolytically coating the terminal face with palladium (palladium coating) in one method step, and electrolytically coating the terminal face with zincate (zincate coating) in an ensuing method step.
The refinement of the nucleation structure achievable with the preceding variant and concurrent refinement of the contact metallization structure can be increased even further by at least partially removing the zinc particles after the zincate coating, and then renewing the zinc coating process.
Hybrid nucleation followed by autocatalytic metal deposition that yields a contact metallization with especially good adhesion to the terminal face given a largely uniform surface morphology for the contact metallization becomes possible if a first electrolytic coating of the terminal face with zincate (zincate coating) takes place in a first method step, the electrolytic coating of the terminal face with palladium (palladium coating) takes place in a subsequent method step, and a second zincate coating takes place in another method step.
This variant also makes it possible to even further improve the surface morphology of the contact metallization by at least partially removing the zinc particles after the first zincate coating and before the palladium coating.
If the terminal face is nucleated in a combined electrolyte comprised of zincate and palladium ions, so that the electrolytic coating of the terminal face with zincate (zincate coating) and electrolytic coating of the terminal face with palladium (palladium coating) take place in a combined method step, so that zincate coating takes place simultaneously with palladium coating due to the combined or mixed electrolyte (zincate/palladium coating), it becomes possible to achieve a well adhering autocatalytically deposited contact metallization as well as a flat surface morphology based on only a single method step for nucleating the terminal face.
In one advantageous modification of this method variant at least a partial removal of the zinc particles with subsequent renewed zincate coating takes place after zincate/palladium coating.
As an alternative, it is also possible to have zinc ate/palladium coating be preceded by a zincate coating of the terminal face, followed by at least a partial removal of the zinc particles.
Stripping the zinc particles has proven to be particularly advantageous for at least partially removing the zinc particles.
Regardless of the method variant selected, the application of the method according to the invention as described above drawing reference to different variants results in a bump which has a basic metallization according to the invention with both zinc and palladium as constituents, wherein at least one constituent of the basic metallization takes the form of numerous mutually independent areas.
In the following, preferred variants of the method according to the invention will be described in greater detail based on the drawings.